QT prolongation is an electrographic repolarization abnormality of the cardiac muscle that can cause syncope, distinctive ventricular tachycardia (e.g., torsades de pointes), and, in rare cases, sudden cardiac death. Many drugs, including antipsychotics, have the potential to prolong the QT interval by blocking the rapidly activating delayed rectifier current (Ikr). Recent evidence suggests that only 5% to 15% of persons who experience drug-induced torsades de pointes carry a mutation in one of the ion channel genes associated with hereditary long QT syndrome (LQTS). It is likely that other genes and environmental factors contribute to drug-induced LQTS. Although there is a poor correlation between the extent of QT prolongation and the occurrence of torsades de pointes, identifying new genetic factors that predispose patients to drug-induced QT prolongation may help in understanding and preventing this type of ventricular tachycardia.